Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!rocksanne!rocksvax!martyl
From: martyl@rocksvax.UUCP
Newsgroups: comp.os.minix
Subject: Controlling processer status -- Minix v. Xinu
Message-ID: <87@rocksvax.UUCP>
Date: Thu, 1-Oct-87 08:01:20 EDT
Article-I.D.: rocksvax.87
Posted: Thu Oct  1 08:01:20 1987
Date-Received: Sat, 3-Oct-87 06:44:34 EDT
Organization: Xerox: Henrietta, NY
Lines: 56

Minix uses two routines -- lock()/restore() which saves the processor 
status in a common private variable (lockvar).

If found this way of coding to often leads to problems when you want to
nest subroutines which each require the interrupts be turned off,
and you don't want to write the code assuming the low-level subroutine is
running with the interrupts turned off.

Xinu uses two subroutines -- disable()/restore().  Disable returns the
processor status and restore gets passed the new processor status -- the
psw should be stored in local memory of the calling function.

I've used variants of the lock/restore/unlock variant for several years and
have gotten bitten too many times than I care to remember.  I'm going to
change all references of lock/restore to disable/restore (I just got bitten
again doing some hacking).

Any comments?

Here's the code for disable/restore (real simple):

; disable interrupts and return previous flags setting
procdef disable
	pushf
	cli
	pop	ax	; to return to caller
	pret
pend 	disable

; restore processor status as it was
procdef restore, <<proc_status, word>>
        push    proc_status
        popf            ; flags are according to proc_status
        pret
	pend    restore

P.S.  The above was written using the Aztec assembler (it has a series of
macros which knows how to get at arguments.  If arguments are passed in, it
knows to do a 
	push	bp
 	mov	bp,sp

and pret does a
	pop	bp

only if bp was pushed).

marty



-- 
marty leisner
xerox corp
leisner.henr@xerox.com
martyl@rocksvax.uucp